In general, in a liquid crystal display device that performs color display, one pixel is divided into three sub pixels of a red pixel, a green pixel, and a blue pixel, the red pixel being provided with a color filter that transmits red light, the green pixel being provided with a color filter that transmits green light, the blue pixel being provided with a color filter that transmits blue light. While color display is possible by use of the color filters provided in the three sub pixels, about two-thirds of light from a backlight which is applied to a liquid crystal panel is absorbed in the color filters. Hence a liquid crystal display device employing a color filter system has a problem of low efficiency in light utilization. Attention has thus been focused on a liquid crystal display device employing a field sequential system which performs color display without using color filters.
In a typical liquid crystal display device employing the field sequential system, one frame period, which is a display period of one screen, is divided into three fields. Although the field is also referred to as a sub frame, the term “field” will be used throughout the following description. For example, one frame period is divided into: a field (red field) that displays a red screen based on a red component of an input image signal; a field (green field) that displays a green screen based on a green component of the input image signal; and a field (blue field) that displays a blue screen based on a blue component of the input image signal. By displaying the primary colors one by one as described above, a color image is displayed on the liquid crystal panel. Since the color image is displayed in this manner, the color filters are not required in the liquid crystal display device employing the field sequential system. Accordingly, the efficiency in light utilization of the liquid crystal display device employing the field sequential system is about three times as high as that of the liquid crystal display device employing the color filter system. The liquid crystal display device employing the field sequential system is thus suited for high luminance and lower power consumption.
It should be noted that, in the present specification, a combination of a data value of the red component, a data value of the green component, and a data value of the blue component is referred to as an “RGB combination.” For example, “R=128, G=32, B=255” is one RGB combination. In this example, a data value of the red component is 128, a data value of the green component is 32, and a data value of the blue component is 255. The data value is typically a gradation value.
Meanwhile, in the liquid crystal display device, an image is displayed by controlling a transmittance of each pixel with a voltage (liquid crystal application voltage). In this regard, it takes several milliseconds for the transmittance at each pixel to attain a target transmittance from the start of writing data (applying a voltage) into the pixels. Hence in the liquid crystal display device employing the field sequential system, in each field, a backlight of the corresponding color is switched from an unlighted state to a lighted state after the liquid crystal has responded to some extent.
Further, in the liquid crystal display device, a sufficient image quality may not be obtained, for example at the time of displaying a moving image, due to a low response speed of the liquid crystal. Then, as one of measures against the low response speed of the liquid crystal, a drive system called overdrive (overshooting drive) has conventionally been adopted. The overdrive is a drive system in which the liquid crystal panel is supplied with a drive voltage higher than a predetermined gradation voltage corresponding to a data value of an input image signal in the current frame or a drive voltage lower than a predetermined gradation voltage corresponding to a data value of an input image signal in the current frame in accordance with a combination of a data value of an input image signal in one previous frame and a data value of an input image signal in the current frame. That is, the overdrive leads to correction of an input image signal that emphasizes (not a spatial change but) a temporal change in a data value. By adopting the overdrive, in the current liquid crystal display device employing the color filter system, the liquid crystal makes a response such that the transmittance almost attains the target value (target transmittance) in each field.
It should be noted that the following prior art documents are known in relation to the present invention. Japanese Translation of PCT International Application Publication No. 2003-502687 discloses an invention concerning an operation to compensate color impurities in a color sequential LCD image display device. According to this invention, a signal of each color is corrected based on a signal of a preceding color. For example, when color display is performed in the order of “blue, green, and red”, a green signal is corrected based on a blue signal. Further, Japanese Laid-Open Patent Publication No. H7-121138 discloses an invention concerning the color reproducibility in a time-division color liquid crystal display device. According to this invention, the scanning timing of a time-division light emitting device of three primary colors is delayed by an optical response speed of liquid crystal, and a non-emission period corresponding to the optical response time of the liquid crystal is provided. Moreover, at the time of writing data into a pixel, gamma correction is performed in accordance with a result of comparison between data in a previous field (a field one field before the current field) and data in the current field. Moreover, WO 2010/084619 A discloses an invention in which the overdrive is applied to the liquid crystal display device employing the field sequential system.